The present application concerns oxygen and/or nitrogen enrichment in internal combustion engines to reduce noxious emissions and in some embodiments to provide enhanced engine performance using a compact and efficient air separation apparatus based on pressure swing adsorption (PSA), preferably with a high frequency cycle.
While diesel engine power trains are highly efficient, they are severely challenged by the urgent need to meet clean air requirements for greatly reduced emissions of unburned hydrocarbons, carbon monoxide, obnoxious and potentially carcinogenic particulate matter, and NOx. The latter two, particulate matter and NOx, are especially challenging for diesel engine power trains. Typically, mitigation measures to reduce NOx tend to increase particulate matter emissions, while measures to reduce particulate emissions tend to increase NOx. Mitigation techniques in use or under development include using cleaner burning fuels, exhaust gas recirculation (EGR), particulate traps, improved after-treatment catalysts, selective reduction catalysts (SCR) using urea, and advanced after-treatment technologies including non-thermal plasma or corona discharge devices.
Some attention has been devoted over many years to the possibility of improving engine performance and addressing emissions problems by modifying the oxygen and nitrogen concentrations of air supplied to diesel and other internal combustion engines.
Oxygen enrichment can greatly reduce emissions of particulate matter, unburned hydrocarbons, CO and smoke, although at the risk of worsening NOx emissions. Some investigators have found that oxygen enrichment may significantly improve engine power density and gross thermal efficiency (before allowing for the parasitic power load of air separation). It has also been found that the tendency toward worsened NOx emissions may be offset in compression ignition engines by retarding the timing of fuel injection, thus achieving with modest oxygen enrichment (e.g. to 25% or less O2) an attractive compromise with an overall modest improvement in all emission categories of incomplete combustion and NOx.
The opposite approach of nitrogen enrichment (e.g. to reduce O2 concentration from the normal 21% to about 19%) has also been advocated as an alternative to EGR, reducing NOx emissions while avoiding the problems of accumulating abrasive or corrosive contaminants from the exhaust.
Examples of this approach include Maissant et al. (French Patent No. 2755187B1), Nakajima et al. (U.S. Pat. No. 3,817,232), Cullen et al. (U.S. Pat. No. 5,678,526), Yi (U.S. Pat. Nos. 5,517,978 and 5,553,591), Manikowski (U.S. Pat. No. 5,706,675), Tsang et al. (U.S. Pat. No. 4,883,023), Poola et al. (U.S. Pat. Nos. 5,636,619; 5,649,517 and 6,055,808), Sekar et al. (U.S. Pat. No.5,526,641), Ng et al. (U.S. Pat. No. 5,640,845), Nemser et al. (U.S. Pat. No. 5,960,777) and Stutzenberger (U.S. Pat. No. 5,908,023).
A further approach advanced in the doctoral thesis of Daniel Mather and subsequently by Chanda et al. (U.S. Pat. No. 6,067,973) is late cycle injection of enriched oxygen to an engine cylinder, so that oxygen admitted late during cylinder expansion may improve the completeness of combustion without raising cylinder temperature high enough to adversely affect NOx levels.
Until now, despite many studies and experimental tests, auxiliary air separation equipment for combustion engines has proved to be impractical, because of excessive power consumption to achieve even a modest change between oxygen and nitrogen atmospheric concentrations. Furthermore, the additional equipment may be too bulky and too costly in relation to any emissions benefit provided.
Previous investigators of air separation for combustion engines have considered several established industrial technologies for air separation, including cryogenic distillation, pressure swing adsorption, and membrane permeation. Cryogenic air separation requires large plant sizes and bulky insulation to approach its best energy efficiency, and has been rejected as completely unsuitable for mobile applications. Conventional pressure swing adsorption processes have a large adsorbent inventory in relation to their productivity, and are prohibitively bulky for mobile applications.
Polymeric membrane systems have been selected by most prior investigators as the most promising available technology, because of their simplicity and relative compactness. However, the compactness of membrane systems is seriously compromised by operation at the relatively low differential pressures that may be considered in engine applications. Power consumption of blowers and/or vacuum pumps for a membrane system is too high in relation to performance benefits expected.
The present processes and systems are concerned with application of a pressure swing adsorption system to air separation auxiliaries for internal combustion engines.
Gas separation by pressure swing adsorption is achieved by coordinated pressure cycling and flow reversals over an adsorber that preferentially adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the mixture. The total pressure is elevated during intervals of flow in a first direction through the adsorber from a first end to a second end of the adsorber, and is reduced during intervals of flow in the reverse direction. As the cycle is repeated, the less readily adsorbed component is concentrated in the first direction, while the more readily adsorbed component is concentrated in the reverse direction.
A xe2x80x9clightxe2x80x9d product, depleted in the more readily adsorbed component and enriched in the less readily adsorbed component, is then delivered from the second end of the adsorber. A xe2x80x9cheavyxe2x80x9d product enriched in the more strongly adsorbed component is exhausted from the first end of the adsorber. The light product is usually the desired product to be purified, and the heavy product often a waste product, as in the important examples of oxygen separation over nitrogen-selective zeolite adsorbents and hydrogen purification. The heavy product (enriched in nitrogen as the more readily adsorbed component) is a desired product in the example of nitrogen separation over nitrogen-selective zeolite adsorbents. Typically, the feed is admitted to the first end of an adsorber and the light product is delivered from the second end of the adsorber when the pressure in that adsorber is elevated to a higher working pressure. The heavy product is exhausted from the first end of the adsorber at a lower working pressure. In order to achieve high purity of the light product, a fraction of the light product or gas enriched in the less readily adsorbed component may be recycled back to the adsorbers as xe2x80x9clight refluxxe2x80x9d gas after pressure letdown, e.g. to perform purge, pressure equalization or repressurization steps.
The conventional process for gas separation by pressure swing adsorption uses two or more adsorbers in parallel, with directional valving at each end of each adsorber to connect the adsorbers in alternating sequence to pressure sources and sinks, thus establishing the changes of working pressure and flow direction. The basic pressure swing adsorption process also makes inefficient use of applied energy, because of irreversible expansion over the valves while switching the adsorbers between higher and lower pressures. More sophisticated conventional pressure swing adsorption devices achieve some improvement in efficiency by use of multiple pressure equalization steps and other process refinements, but complexity of the valve logic based on conventional 2-way valves is greatly increased. Furthermore, the cycle frequency with conventional valves and granular adsorbent cannot be greatly increased, so the adsorbent inventory is large. Conventional PSA plants are accordingly so bulky and heavy that their use to enrich oxygen or nitrogen for internal combustion engines may be less than ideal, particularly for any vehicle applications.
By operating with high-surface-area, laminated adsorbers, with the adsorbent supported in thin sheets separated by spacers to define flow channels between adjacent sheets, and with the adsorbers mounted in a rotor to provide the PSA process valve logic with only one moving part, a high frequency PSA cycle (e.g., at least 25 cycles/minute) can be performed in an extremely compact apparatus as disclosed by Keefer et al (Canadian Patent application Nos. 2,312,506, 2,274,286 and 2,274,318). Alternatively, a PSA unit that achieves more than 1 PSA cycle per rotor revolution could be used in the presently disclosed processes and systems. Particular embodiments of the disclosed processes and systems provide for using such compact PSA devices in conjunction with internal combustion engines to provide oxygen and/or nitrogen enrichment in order to address the problems of emissions of unburned hydrocarbons, particulate, carbon monoxide, and NOx; while also to achieve favourable power density and overall efficiency.
Increasing the oxygen flow to the engine offers the benefits of reduced particulate emissions and increased engine gross power output, while also facilitating ignition of lower-grade fuels.
Increasing the nitrogen concentration of air fed to the engine potentially reduces nitrogen oxide emissions without the problems caused by exhaust gas recirculation (engine wear, oil contamination).
According to one disclosed embodiment, a process and system is described for providing fuel and an oxygen-enriched stream to at least one chamber of an internal combustion engine, comprising providing at least one pressure swing adsorption module that produces an oxygen-enriched stream; providing an internal combustion engine chamber that includes a first combustion zone and a second combustion zone; providing a fuel-rich mixture of the oxygen-enriched stream and a first fuel in the first combustion zone of the internal combustion engine chamber; and providing a fuel-lean mixture of air and a second fuel in the second combustion zone of the internal combustion engine chamber. According to one variant, the process is directed to igniting a gaseous fuel in an internal combustion engine and includes providing a fuel-lean mixture of air and gaseous fuel in a primary combustion zone of the internal combustion engine chamber.
According to another embodiment, a process and system is described for providing an oxygen-enriched stream to at least one internal combustion engine chamber, comprising providing an oxygen-enriched stream (such as by using a PSA unit); recycling exhaust gas from an internal combustion engine chamber via a closed loop; mixing together the oxygen-enriched stream and the recycle exhaust gas to produce an engine chamber feed stream, wherein the oxygen-enriched stream is the only source of fresh oxygen in the engine chamber feed stream; and introducing the engine chamber feed stream into the internal combustion engine chamber.
A further disclosed internal combustion engine process and system involves providing an oxygen-enriched stream (such as by using a PSA unit); providing a first internal combustion engine chamber that produces a first engine exhaust stream; mixing together the oxygen-enriched stream and the first engine exhaust stream to produce a feed steam; and introducing the feed stream into a second internal combustion engine chamber.
An additional embodiment relates to a process for providing an oxygen-enriched stream to at least one internal combustion engine chamber, comprising introducing a first air stream into an internal combustion engine chamber having a displacement element (e.g., a piston or a rotor); introducing a second air stream into a pressure swing adsorption module to produce an oxygen-enriched stream and a nitrogen-enriched stream; introducing the oxygen-enriched stream into the internal combustion engine chamber during an expansion stroke of the displacement element; and introducing the nitrogen-enriched stream into the internal combustion engine chamber during an intake stroke of the displacement element.